The present invention relates to a recording apparatus for recording an image by drawing dots on a recording medium and, more particularly, to a recording apparatus which can achieve high image quality.
Recording apparatuses arranged in a printer, a copying machine, a facsimile apparatus, and the like record an image consisting of a dot pattern on a recording medium such as a paper sheet, a resin thin plate, or the like on the basis of input image information. Such recording apparatuses can be classified into a wire-dot system, a thermal transfer system, a thermal recording system, an ink-jet system, an electrophotography system, and the like depending on their recording methods. In recent years, of these systems, the thermal transfer system and the ink-jet system are used in many recording apparatuses since they can realize high resolution with low cost.
A recording apparatus of the above-mentioned sort forms dots (a to h) using a recording head 1 on which recording elements (A to H) having image forming portions, as shown in FIG. 33. This operation is repeated upon recording scanning (main scanning) of the recording head 1, and each of the recording elements further forms a plurality of dots ((1) to (5) in FIG. 33), thereby recording an image.
However, since the above-mentioned recording apparatus uses a plurality of recording elements, a variation in dot area, a variation in dot formation position, and the like occur depending on the recording elements. For this reason, upon recording of image information which requires uniform dot areas and a uniform dot arrangement, a stripe-shaped nonuniformity is generated. This phenomenon will be described below with reference to FIGS. 34 to 36.
FIG. 34 shows an ideal state of a recorded image obtained by recording dots with uniform areas at accurate positions on the basis of basic 50%-halftone image information. FIG. 35 shows a 50%-halftone recorded image obtained when the area of each of dots in a line c recorded by the recording element C is larger than a standard area, and the area of each of dots in a line f recorded by the recording element F is smaller than the standard area. The image constituted by such dots becomes darker near the line c, and also becomes lighter near the line f. Since this phenomenon continuously occurs, a stripe-shaped nonuniformity (banding) is generated in the recorded image.
A 50%-halftone recorded image shown in FIG. 36 has uniform dot areas recorded by all the recording elements (A to H). However, dots in a line b recorded by the recording element B and dots in a line d recorded by the recording element D are offset from the standard positions toward the line c. In this case, the interval between dots b and c and the interval between dots c and d become smaller than a normal interval (lines e to f), and these portions become darker. Also, since the interval between dots a and b and the interval between dots d and e become larger than the normal interval, these portions become lighter. For this reason, a stripe-shaped nonuniformity is generated in the recorded image.
The above-mentioned phenomenon occurs due to a slight structural variation in recording elements when a high resolution is to be attained, and it is difficult to eliminate this phenomenon by improvements in the manufacture. It is also very difficult to control a variation in recording elements caused by aging of the recording elements, and to keep all the recording elements in the same condition.
The stripe-shaped nonuniformity which deteriorates image quality is generated since "light" and "dark" image portions are continuously formed in a recorded image. For this reason, a control method (random dot (size) control) for forming dots using the recording elements while irregularly changing their dot areas, so as to eliminate the nonuniformity of an image as a whole, although individual dot sizes vary, has been proposed.
However, the method of forming dots while irregularly changing the dot areas suffers the following problems.
As the first problem, processing power required for a CPU of the recording apparatus undesirably increases. For example, in a serial printer for performing recording by scanning a carriage in units of lines, a recording head normally has nozzles more nozzles than are required for printing one line. For example, a 360-DPI recording head has more than 60 nozzles. If the ejection driving speed is 60 kHz, such a recording head performs 360,000 heats (ejection operations) per second. Therefore, in order to irregularly change dot areas, a random number must be generated at least 360,000 times per second, and heat pulses to be applied to the nozzles must be switched in correspondence with the random number values.
Furthermore, in order to satisfy the recent requirement for achieving a high recording speed, a serial printer which can record several lines per scan and a recording head which can be driven at a high-speed period have been developed. Therefore, the processing power of the CPU required in random number control further increases.
As the second problem, a problem associated with adverse effects on image quality is posed. When the dot size is randomly changed, generation of a regular nonuniformity (e.g., the above-mentioned white/black stripe-shaped nonuniformity), which is conspicuous to the human eyes as a pattern, can be eliminated. However, the entire image may become noisy depending on the pattern. Such a noisy image is formed depending on the modulation width upon modulation of random dot sizes, and is one of adverse effects of the random control.